I slept flat on my back until my body told me I shouldn't anymore. I did with both my pregnancies up until about 17 or 18 weeks if I recall. When it gets uncomfortable, stop. I'm 34 weeks now and physically have to semi wake up to sit up and roll over. If I can't get comfortable I just prop myself up and sleep that way. You do what works for you. i'm sure you'll stop waking up on your back when you get farther along. Baby is super heavy and there isn't any ignoring him/her smooshing your insides;) no need to worry!
You CAN sleep on your back. However the key is to sleep with your upper back and head elevated by 2-3 pillows (kind of like a ramp). Whatever you do DO NOT LAY FLAT on your back. I'm 23 weeks along and my legs and butt fall asleep when on my side. I have to sleep facing up with plenty of pillows keeping my back and head elevated. Also try using a pregnancy/maternity pillow. Those keep u pretty stable while u sleep so u can't rollover so easily.
Pillow seems like a great idea. My sister is 17 weeks and I'm 11 but she was waking up on her back also and told her doctor. Her doctor made it clear to never sleep on your back while pregnant epically in 2nd and 3rd trimester cause it can cause fainting while your asleep. And her doctor didn't sugar coat it as she also told her some women have fainted and never woke up, which is very sad and scary to think about. Hope the pillow works out for you.
The main issue with sleeping on your back comes into play as your uterus grows and gets heavy. You have an artery that runs through that area and the pressure from your uterus can cause constriction. What I did was put a big pillow behind me so that if I tried to roll back onto my back, that would catch me and I'd never end up on my back. good luck